Maximum Depth of binary tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
int maxDepth(TreeNode *root) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if (!root){
return 0;
}
return max(maxDepth(root->left),maxDepth(root->right)) + 1;
}
};